Add changelog

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased](https://github.com/open-telemetry/opentelemetry-python/compare/v1.4.0-0.23b0...HEAD)
+- Add regexes to check W3C baggage keys and values
+  ([#2017](https://github.com/open-telemetry/opentelemetry-python/pull/2017))
 - `opentelemetry-distro` & `opentelemetry-sdk` Moved Auto Instrumentation Configurator code to SDK
   to let distros use its default implementation
   ([#1937](https://github.com/open-telemetry/opentelemetry-python/pull/1937))